Mammalian spermatozoa have been known to be antigenic for many years. More recently, it has been demonstrated that mammalian sperm contain an antigenic enzyme, which is known as the C.sub.4 isozyme of lactate dehydrogenase (LDH-X, LDH-C.sub.4). LDH-C.sub.4 has been isolated in pure crystalline form from mouse testes. Goldberg (1972) J. Biol. Chem. 247:2044-2048. The enzyme has a molecular weight of 140,000 and is composed of four identical C subunits. The amino acid sequence and three-dimensional structure of LDH-C.sub.4 has been studied and partially determined by a number of investigators. See Musick et al (1976) J. Mol. Biol. 104:659-668; and Wheat et al (1977) Biochem. & Biophys. Res. Comm., 74, No. 3:1066-1077. Wheat et al determined the sequence of the essential thiol peptide from amino acid 159 to 171, and found this to be nearly identical to essential thiol peptides from other vertebrate LDH isozymes.
In 1974, Dr. Erwin Goldberg reviewed the effects of immunization with LDH-X (LDH-C.sub.4) on fertility, and advanced the possibility that "by using a defined macromolecular constituent of sperm it becomes possible to elucidate its primary structure in terms of amino acid sequence, to map specifically the antigenic determinant (s) responsible for inducing infertility, and then to construct synthetic peptides containing these determinants. Possessing the capability for synthesizing a molecule with such properties, makes the immunological approach to fertility control feasible." Karolinska Symposia on Research Methods in Reproductive Endocrinology, 7th Symposia: Immunological Approaches to Fertility Control, Geneva, 1974 202-222. However, such synthetic antigenic peptides remained a goal and not an achievement, although their theoretical desirability had been recognized. In 1979, Dr. Erwin Goldberg summarized the state of the art as follows:
"In conclusion, and on a practical basis immunotherapy for birth control requires more than effectiveness, specificity, reversibility, and absence of systemic side reaction. Rather large amounts of the antigen must be available in unequivocally pure form. This condition probably cannot be met by a natural product enzyme antigen from sperm or testes. Rather, contraceptive technology requires a synthesizable peptide fragment retaining antigenicity and provoking a response which impairs fertility. Completion of the structural analysis of LDH-C.sub.4 should allow mapping of antigenic determinants and synthesis of such peptides for use in a new contraceptive technology," Recent Advances in Reproduction and Regulation of Fertility, G. P. Talwar, editor, Elsevier/North Holland Biomedical Press (1979).